My experience with the "Roll over" is that it can be hard for some dogs to do physically. My guy is very long and lanky and I think he has a really hard time with it, so then he gets kind of anxious that he can't do it and does exactly what you describe. If you have a small hill anywhere, try it there-- have gravity give him a hand
That was how I taught him initially, so I think struggling to do it on flat ground was hard for him.